1. Field of the Invention
The invention relates to physical security systems utilizing a pressure sensitive optical fiber transducer system wherein the arrangement of optical fibers is combined with signal processing capabilities.
2. Description of the Prior Art
Physical security systems utilizing fiber optics are well known in the prior art and can be broken down into two designations, namely, point sensors and line sensors. A point sensor system can be described as one having a device which measures a disturbance at one point or several discrete points and transmits signals via an optical fiber to a control room or alarm system. In such a system a disturbance at certain discrete points is sensed by means of electrical or optical switches and the like. The optical fiber is used merely to transmit evidence of the disturbance to a detector device. A line sensor system is one in which a disturbance can be detected anywhere along the whole length of the fiber wherein the fiber itself is the sensing element.
U.S. Pat. No. 4,275,296 to Adolfsson, U.S. Pat. No. 4,379,289 to Peek, U.S. Pat. No. 4,281,245 to Brogardh et al. and U.S. Pat. No. 4,367,460 to Hodard are examples of point sensor systems employing optical switches. These types of security systems are not suitable for perimeter or large area protection because of the large number of individual transducers and/or switches that are necessarily required therefor.
The line sensor systems used in above ground applications are exemplified by U.S. Pat. No. 4,275,294 to Davidson and U.S. Pat. No. 4,370,020 to Davey wherein the optical fibers are strung out so that breakage or severe distortion anywhere along the length of the fiber by an intruder will be detected.
Obvious disadvantages of these type systems include visibility of the fibers to an intruder, destruction of the fiber when an intrusion results and exposure to extraneous elements such as weather and the like. Line sensor systems utilizing underground installation of a fiber optic cable as evidenced by U.S. Pat. No. 4,321,463 to Stecher and U.S. Pat. No. 4,297,684 to Butter overcome the above noted disadvantages of the above the ground type. Such systems are generally constructed with the sensing changes in the phase measurements of the light transmitted in the optical fiber and as a result are extremely costly, inflexible in terms of actual installation and subject to a high likelihood of a false alarm.
No known prior art physical security system measures changes in the intensity of light transmitted through fiber optic strands. The only technology known to applicants related to measurement of change in intensity of light in fiber optic strands is embodied in the teachings of U.S. Pat. No. 4,342,907 to Macedo et al. In that patent it is noted that a cladded optical fiber subjected to pressure has inherent therein a change of the refractive index in the cladding material. The change of the refractive index results in a change in the light being transmitted in the optical fiber strand because light is allowed to escape from the fiber core.
Heretofore no one has been able to utilize the principle taught in the Macedo et al patent in a physical security system.